1. Field of the Invention
The present invention relates to variable stage charge pumps, and, more specifically, to a charge pump that has a selectable number of stages.
2. Description of the Related Art
In some electronic devices, the power supply voltage is not high enough to guarantee the correct operation of the circuit. In these cases, higher voltages are required. For example, voltages up to 15 volts are needed for programming and erasing operations in a non-volatile memory device.
When a voltage higher than the power supply voltage is required, a second power supply can be furnished from outside the circuit. Until a few years ago, this was the only solution available. Presently, the required voltages can be also generated inside the integrated circuit. On-circuit high voltage generation is now more widespread than off-chip generation, because of lower implementation cost.
A circuit that can provide higher voltage values than the power supply is typically called a charge pump.
FIG. 1 shows, in a simplified manner, an N-stage charge pump 10, each stage comprising a switch 12 and a pumping capacitor 14. Additionally a load capacitance 16 and a load current 18 are shown. The integration area of the charge pump is very large and it increases when the current output must be high.
The current output from a charge pump can be determined as follows:                               I          L                =                                                                              (                                      N                    +                    1                                    )                                ·                                  V                  dd                                            -                              V                OUT                                                    N              2                                ·                      C            TOT                    ·          f                                    (        1        )            where,
N is the number of charge pump stages;
Vdd is the power supply voltage;
Vout is the output voltage;
CTOT is the total capacitance value of all pumping capacitors; and
f is the switching frequency.
In order to make the output current equal to the current required by the load, the pumping frequency f, is varied from 0 Hz to fMAX.
Therefore, the maximum current value for an N-stage charge pump is:                               I                      L            ⁢                                                   ⁢            MAX                          =                                                                              (                                      N                    +                    1                                    )                                ·                                  V                  dd                                            -                              V                OUT                                                    N              2                                ·                      C            TOT                    ·                      f            MAX                                              (        2        )            
The power supply voltage Vdd can be of any value within a given fixed range. The output voltage Vout is related to the load driven in a given time by the charge pump. The capacitors that make up CTOT require the largest part of the integration area of the charge pump
In order to maximize the current output from the charge pump, with the same amount of occupied area (and thus the same CTOT) and with the same maximum frequency, fMAX, the number of stages N can be opportunely chosen.
As described in Optimization of Word-Line Booster Circuits for Low-Voltage Flash Memories—IEEE Journal of Solid-State Circuits, Vol. 34, No. 8, August 1999, the number of stages N for which the current output has the maximum value is:                               N          OPT                =                  2          ·                      (                                                            V                  OUT                                                  V                  dd                                            -              1                        )                                              (        3        )            
As seen from equation 3, the number of stages is dependent on the power supply voltage, Vdd, and the output voltage VOUT. The supply voltage may not be known at the time the charge pump is designed, and output voltage, VOUT have different requirements at different times, for example in order to drive different loads. Both of these unknown values makes it difficult to accurately design a suitably sized charge pump. If the number of stages chosen is too few, the necessary output current may not be able to be achieved; if the number of stage chosen is too many, valuable integrated circuit area (for the unnecessary extra capacitors) is wasted.